Allogeneic marrow transplantation has been a successful therapeutic approach to treat (and cure) genetic disorders which involve hematopoietic cells. However, there are several problems with this approach. For example, appropriate donors are not available for many individuals who could benefit from transplantation; opportunistic infections, graft vs. host disease (GVHD), and secondary malignancies can result from the preparative myeloablative therapies; and in many genetic diseases, such as lysosomal storage diseases, liver and neurologic toxicity occurs by birth, which is not reversible with subsequent transplantation. For these reasons, we plan to test the feasibility of in utero transplantation in a large animal model, as a preclinical study. Geoffroy cats and domestic cats are physiologically, genetically, and hematologically similar, but cells derived from these animals can be distinguished by analyses of their glucose-6-phosphate dehydrogenase (G-6-PD) phenotypes and by the presence or absence of endogenous retrovirus (RD1 14) sequences. These assays are sensitive, specific, and easy to quantify. The first goal of this application is to optimize methods for the in utero transplantation of hematopoietic stem cells from adult Geoffroy cats into domestic cat fetuses. Several strategies for the enrichment of feline hematopoietic stem cells and for the depletion of mature T cells (to avoid GVHD) are proposed. The second goal of this application is to apply these methods to treat alpha mannosidosis, a naturally occurring lysosomal storage disease in cats. This disorder has a clear clinical progression which results in death by 6 months of age and widespread neurologic storage. Although post- natal marrow transplantation ameliorates the disease course, storage within CNS macrophages and perithelial cells is unaffected. Therefore, an in utero transplantation approach is particularly attractive. The proposed studies should demonstrate the feasibility and clinical utility of in utero hematopoietic stem cell transplantation in the therapy of genetic disorders.